1. Field of the Invention
The present invention relates to a fluid pressure servo system and, more particularly, to a system designed to maintain a constant spacing as between a member and a surface, separated by a hydrodynamic fluid.
2. Description of the Prior Art
In the development of a video disc system, information is recorded on a disc as a series of irregularities, each approximately 1 micron in width, arranged in a line with adjacent lines or tracks being spaced apart by approximately 2 microns center to center. As noted in the above-identified Elliott application, the problem of optically resolving irregularities of these dimensions requires that the focus of the optical system be maintained constant to within a micron.
One method of maintaining a constant disc-to-head spacing which could be adapted to optical reading of a video disc is described in U.S. Pat. No. 3,132,328, issued May 5, 1964 to M. K. Taylor, entitled "Reading and Writing Head Mounting Means". In this patent, a double bellows arrangement is used in conjunction with a control nozzle to keep the head to disc spacing constant. The control nozzle blows air against the disc and causes a predetermined nozzle pressure. If the nozzle pressure increases because the head is too close, the pressure causes a control bellows connected thereto to enlarge and to push against a compensating bellows which is maintained at a constant line pressure to force the head away from the disc. If the nozzle pressure decreases because the head is too far away, however, the pressure in the control bellows is not decreased but rather the compensating bellows exerts a force greater than the force exerted by the control bellows and forces the head closer to the disc. The above arrangement, however, has the disadvantage that it requires a constant line pressure since the compensating bellows must exert a precise force on the control bellows and must be in equilibrium with the control bellows at a predetermined head-to-disc spacing.
As described in the above-identified Canino application, another method of reading the information recorded on a disc is through the use of a transducer head assembly placed adjacent the disc and which, along with the disc, is immersed in a fluid thus creating a hydrodynamic fluid bearing between the assembly and the disc. As is further described in the Canino application, the spacing maintained between the assembly and the disc can be kept substantially constant by means of a source of fluid at a variable controllable pressure and a pressure regulator including a fluid flow restrictor which responds to changes in separation between the assembly and the disc and modifies the fluid pressure applied to the head. In the Canino application, however, the head assembly and the pressure regulator are two separate structures and the resultant combination does not lend itself readily to miniaturization, a feature greatly desirable in present day video equipment. In addition the pressure regulator includes a valving arrangement for varying the fluid pressure which could further complicate an attempt at miniaturizing and combining the two separated structures.